Our goal is to learn how neurotransmitters regulate cardiac functionduring embryogenesis. We shall test a cyclic AMP hypothesis that has been applied to the embryonic heart to explain the actions of excitatory (Beta-adrenergic) and inhibitory (muscarinic) drugs that mimic autonomic transmitters. The developing avian heart is an instructive model for the study of neurotransmitter action. After hatching, acetylcholine inhibits ventricular function directly. In the embryo, the ventricle displays the properties of the adult mammalian ventricle insofar as acetylcholine has no effect on function unless the tissue is stimulated by stbstances (isoproterenol, isobutylmethylxanthine, cholera toxin) that cause cyclic AMP to accumulate. The generation of Ca2+-dependent action potentials and contractility are indices of ventricular cell function and of neurotransmitter action. The specific aims are to study how neurotransmitter action is accomplished by 1.) changing the rate of cyclic AMP synthesis and degradation and by 2.) changing the effects of accumulated cyclic AMP. The third specific aim is to study regulation of membrane Ca2+ channels by cyclic AMP which is changed either acutely by drugs or chronically by developmental variations in cyclic AMP metabolism. Adenylate cyclase and cyclic AMP phosphodiesterase activity of ventricular homogenates and cyclic nucelotide content of ventricular muscle will be measured in the absence and presence of drugs that mimic or alter the actions of parasympathetic and sympathetic transmitters. Taken altogether, the results of these biochemical, pharmacological and physiological experiments, obtained at selected times during embryogenesis, will test the validity of the cyclic AMP hypothesis for neurotransmitter action and interaction (cholinergic-adrenergic antagonism) in the heart. A study of the incorporation and development of elements involved in cyclic AMP-dependent mechanisms for the autonomic neurotransmitter action can provide a better understanding of their operation inadult animals and provide insight into how an imbalance of neurotransmitter activity can elicit changes in membrane electrical activity (arrhythmias) and in contractility (mechanical dysfunction).